The present invention relates generally to the art of compressing a gas in a rotary screw compressor. More specifically, the present invention relates to a method of determining whether or not a slide valve of a screw compressor, in a water chiller system, is in abutment with the compressor slide stop and therefore, whether the compressor is operating in a fully loaded condition.
Compressors are employed in refrigeration systems, known as chillers, to raise the pressure of a refrigerant gas from a suction pressure to a higher discharge pressure which permits the ultimate use of the refrigerant to accomplish the cooling of the desired medium. Screw compressors employ complimentary male and female screw rotors disposed within the working chamber of a rotor housing to compress gas. The screw rotor housing defines suction discharge ports which are in flow communication with the working chamber of the rotor housing
Refrigerant gas at suction pressure enters the compressor working chamber via a suction port at the low pressure end of the rotor housing and is there enveloped in a pocket formed between the rotating complimentary screw rotors The volume of this chevron-shaped pocket decreases and the pocket is displaced toward the high pressure end of the compressor as the rotors rotate and mesh within the working chamber. The gas within such a pocket is compressed and heated by virtue of the decreasing volume in which it is contained, prior to the pocket's opening to the discharge port at the high pressure end of the compressor. The gas pocket, as it continues to decrease in volume, eventually opens to the compressor discharge port at which time the compressed gas is discharged from the working chamber of the compressor.
One advantage of screw compressors resides in the ability to easily modulate their capacity and therefore the capacity of the system in which the compressor is employed. Such capacity control is normally accomplished through the use of a slide valve assembly. The valve portion of the slide valve assembly is built into and forms an integral part of the rotor housing of the compressor and the valve portion of the assembly generally cooperates with the remainder of the compressor's rotor housing to define the working chamber within the compressor. The slide valve is axially movable to expose the screw rotors disposed in the working chamber of the compressor to a location within the compressor, other than the suction port, which is at suction pressure.
The portion of the working chamber initially opened to suction pressure by the movement of the slide valve is that portion immediately downstream of the point at which the compression of refrigerant gas would normally begin within the working chamber. As the slide valve is opened further, a greater portion of the working chamber and the screw rotors therein are exposed to suction pressure. Capacity reduction is therefore obtained by effectively reducing the portion of each rotor used for compression.
When the slide valve is closed, i.e. when it abuts an internal slide stop so as to isolate the rotors from suction pressure other than through the suction port, the compressor is fully loaded and operates at full capacity to compress refrigerant gas. When the slide valve is fully open, that is when the portion of screw rotors exposed to suction pressure other than through the suction port is greatest, the compressor is unloaded to the maximum extent possible.
The positioning of the slide valve between the extremes of the full load and full unload positions is accomplished without difficulty with the result that the capacity of a screw compressor, and the system in which it is employed, is modulated smoothly and efficiently over a large operating range. The slide valve is most often and preferably hydraulically operated by the porting of oil to a piston/cylinder arrangement which is part of the slide valve assembly.
Heretofore, the positioning of the slide valve of some screw compressors employed in water chillers has been a function of the chiller leaving water temperature. That is, irrespective of the actual position of the slide valve in the compressor, its position is modulated to more fully load or unload the compressor in accordance with the difference between actual chiller leaving water temperature and a setpoint temperature so as to produce the amount of refrigeration necessary to produce water at the setpoint temperature. This control scheme is particularly appropriate for water chillers employing a single screw compressor where, once the compressor is fully loaded, no additional refrigeration capacity is available.
However, certain newer water chillers employ more than one screw compressor which requires that a determination be made as to when to energize or de-energize a second compressor in accordance with the need for more or less refrigeration capacity as the case may be. If an operating compressor, in a multiple compressor system, is not fully loaded and therefore has further refrigeration capacity, the slide valve of that compressor can be moved to more fully load that compressor as opposed to energizing another compressor which would be wasteful of energy. However, if the first compressor is operating fully loaded, i.e. the slide valve of that compressor is in abutment with the slide stop, the energization of a second compressor will be required to gain more refrigeration capacity.
In previous screw compressor chillers particularly single compressor water-cooled chillers, the actual movement of the compressor slide valve was readily and reliably indicated by the measurable increase or decrease in compressor motor current draw which coincided with the increase or decrease in load on the compressor which resulted from slide valve movement. However, in the newer chiller systems referred to in the immediately preceding paragraph which can be multiple screw compressor air-cooled water chillers, the change in measured compressor motor current draw, even after several incremental changes in slide valve position, is not necessarily a reliable indicator of the actual movement of the slide valve. This is because chiller system voltage changes, the energization or de-energization of fans, the movement of components such as electronic expansion valves changes in water temperatures and the like can alone or in combination all affect the current drawn by the motor of an operating compressor.
Therefore, in order to use motor current draw as a reliable indicator of slide valve position, and more particularly, as an indicator that the slide valve of a compressor is in abutment with the slide stop which further indicates that the compressor is operating at full load, a discrete method to positively identify such condition by employing compressor motor current draw is required.